modp_b64: Fix BUILD.gn to be able to build the library with GN am: 8a6dfb4ec3 am: b0bff9549b am: c5eaf1bed0
am: fc053cb093

* commit 'fc053cb0934e451ddb3514ec0fb153760ac11853':
  modp_b64: Fix BUILD.gn to be able to build the library with GN

Change-Id: Ie8dbe4404a8c9863bad81540842a2c560355ab69